A mud cake is formed during drilling with the drilling mud injected onto the well via drill string and drawn off via the openings in the drill bit to provide lubrication during the drilling and drilled-out rock debris wash-over to the surface. The mud cake layer is formed as the drilling mud is mixed with the rock debris and/or other solid substances and circulated upwards via the annular area between the drill string outer surface and borehole wall. The mixture covers the borehole wall and forms the mud cake layer. One of the functions of the mud cake layer is insulation of the bed from the borehole inner part. The mud cake layer is in the industry often referred to as the mud cake or filter cake.
A method for direct determination of mud cake characteristics during sampling performed during the drilling process is known. The method is described in WO 2009/139992. In the known method pulse-echo measurements were performed using a high-frequency acoustic detector, and to evaluate the mud cake thickness data of the signal emitted and echo returned were used. To determine the mud cake thickness as per this method the sound speed in the mud cake must be known which in practice is usually impossible.